Commutation system



Sept. 30, 1941. Y D. s. MUZZEY, JR 21,914,

I COMMUTATION SYSTEM' Original Filed Feb. 5, 1938 4 Sheets-Sheet 1 Invemor: David 5. uzzeg Jr. Bg his Attorney; Q

p 3 1941- D. s. MuzzEY. JR 41c. 21,914

COMMUT ATION SYSTEM Original Filed Feb. 5, 1938 4 Sheets-Sheet 2 I Inventor: David S. Muzzeg Jr. 7 B9 his Afiorneg: A

Sept. 1 1- D. s. MUZZEY, JR Re. 21,914 I COMMUTATION SYSTEM Original Filed Feb. 5, 1938 4 Sheets-Sheet 5 Poienfidl Commufuior 7 A Currenf and Pofeniial Pofenfial Commuidior Commuiafor Brushes aligned Brushes advanced Fig. 3.

lnvenior; David Muzzeg Jr.

by hisAHorneq:

COMMUTATION SYSTEM Original Filed Feb. 5, 1938' 4 Sheets-Sheet 4 I i 1 I A F- i i i I 14; 5| 53 I 55 56\ Brush H 9 IO l2 I5 I I3 Brushes v Currenr Commuiafqr i Porenf ial C ommufaior I Fig. 5.

Invenfor: David 5. Muzzeg Jr.

By his Affornegr Reissued Sept. 30, 1941 David Savilie Muuey, In, Houston, Tex, assignor to Shell Development Company, San Francisco, Calif., a corporation Delaware Original No. 2,204,436, dated June 11, 1940, Serial No. 188,951, February 5, 1938. Application for reissue May 19, 1941, Serial No. 394,173

16 Claims.

The present invention pertains to a low-frequency system of synchronous commutation.

Although this invention may be used in electrical circuits of any type, it has a particularly useful application in methods oi geophysical exploration of underground formations, wherein a commutated electric current is passed through the earth strata between grounded current electrodes, while the potential diflerence generated in the earth by said current, or due to other causes, is measured between grounded potentialelectrodes by means of suitable indicating devices.

In exploring the ground bymeans of commutated direct current it is customary to use socalled synchronous commutators, adapted to reverse in synchronism, and at a desired frequency, the connections between the source of current and the current electrodes, and the connections between the indicating devices and the potential electrodes.

Although the methods ior'commutating direct current commonly used in geophysical exploration give satisfactory results when relatively high frequencies, such, ior example, as 40 cycles per second, are used, and when the electrodes are placed relatively close to each other, these methods give rise to various undesirable electrical phenomena in cases when very low frequencies,

such, for example, as one cycle per second, are

used, or when the electrodes are spread over a relatively large distance, such for example, as a mile.

It is therefore the object of this invention to provide a system of synchronous commutation eflective for any desired frequency and any desired electrode spread.

This and other objects of this system as well as the arrangement of apparatus used in practicing the present invention, will be understood from the iollowing description, taken with reference to the attached drawings, wherein:

Fig. 1 is a schematic diagram oi a conventional synchronous commutator;

Fig. 2 is a schematic diagram of the synchronous commutator oi the present invention:

Fig. 3 is a schematic diagram illustrating the control of the time lag between the closing of the-current and of the'potential circuits of the present commutator;

Fig. 4 is'a perspective view of the present commutator;

Fig. 5 is a cross-section view of the present, commutator;

Fig. 6 is a cross-section view taken along line 1-1 of Fig. 5:

Fig. 7 is a cross-section view taken along line 11-11 0: Fig. 5.

Fig. 8 is a cross-section view taken along line IIII1I of Fig. 5; and

Fig. 9 is a cross-section view taken alon line IVIV of Fig. 5.-

The nature of the present invention can be more readily understood by referring first to a conventional type of synchronous commutator schematically shown in the diagram of Fig. 1.

A rotatable shaft i has rigidly mounted thereon an insulated commutator ring ZAB- having segments 2A and 2B insulated from each other,

and a second commutator ring 'IAB having insulated segments IA and 1B. The gaps between the segments of both commutators are accurately aligned with each other. Rigidly mounted on the same shaft are also sllp, rings 3, 4, 5 and I. The commutator segment 2A is electrically connected to slip ring 4 by means of a conductor Ii, while the segment 23 is connected to slip ring I by means of conductor II. In the same manner, the commutator segments 1A and 1B are connected to slip rings 5 and 6 by means or conductors II and I9, respectively.

The commutator ring 2A3 is in contact with brushes 8 and l, which are in circuit with a source of direct current 2| and an indicating device such 'as an "ammeter 22. Slip rings 3 and I are in contact with brushes I0 and II which are connected to the current electrodes 2i and 2|, respectively.

The commutator ring MB is. in contact with brushes l4 and Ii, which are connected with a electrodes are usually arranged in such manner,

that the spread or the Potential electrodes 33 and It overlaps that oi the current electrodes II and It, or includes thelatter, or is included thereby.

With the arrangement of apparatus shown in Fig.1, it willbe seen that a rotation of the shalt I will cause a positive and a negative potential to be alternately applied to electrode 24 through brush i I, slip ring 4, conductor ii and segment 2A,.as brushes I and I alternately come 5 in contact withsaidsesment is, while a corresponding reverse sequence of changes of potential is impressed on the electrode 23 through brush Ill, slip ring 3, conductor l1 and seg- -ment 2B.

A commutated current of a frequency determined by the speed of rotation of shaft I will therefore flow in the ground between the electrodes 23 and 24. The potential diiference generated in the ground between the electrodes 33 and 34 by the commutated current flowing between the electrodes 23 and 24 will alternate in synchronism with the rotation of commutator 2AB. Since, however, the electrodes 33 and 34 are connected through brush i2, slip ring 5,

conductor l8 and segment 1A1, and through brush l3, slip ring 6, conductor is and segment 13, respectively, to brushes i4 and I5, and since commutator ring 1113 rotates in synchronism (1) If it is desired that the current penetrate the ground to a considerable depth, the spacing for the use of low frequencies for the following reasons.

The use of the conventional narrow gaps between the commutator segments causes thebrushes to short-circuit these segments at the moment of commutation. With regard to the current circuit, if the speed of rotation of the commutator 2AB is relatively high, for example, 40- revolutions per second, this brief short-circuiting will have no damaging eifect on the generator 2|, while the effect on the reading of ammeter 22 will be negligible due to the combined inductances of the generator and the ammeter, and the inertia of the latter. A similar situation will obtain in the circuit of the potential commutator 'IAB, where the short-circuiting of the segments IA and 'IB by the brushes I4 and I5 will cause the potential drop of the resistance to be applied to the galvanometer .36. However, with a high speed of commutation, the relatively slow response of the galvanometer will prevent any appreciable error from this cause.

It is, however, obvious that if the frequency of commutation is reduced to some very low value, such, for example, as two, one, or one-half cycles per second, the extremely low speed of rotation between the electrodes must be made large, and

very sensitive measuring devices, or currents of greater intensity, or both, must be used to make measurements sufllciently accurate to detect deep anomalies. As the electrode spacing and the current intensity are increased, the direct electromagnetic coupling between the current and the potential circuits is increased due to the increase in the size of the loops formed by the cables to the electrodes and the path through the ground. At some spacing whose magnitude depends on the arrangement of electrodes and cables and on the sensitivity of the apparatus, the transient electrical phenomena caused at each commutation by this direct coupling becomes-sufficiently great to cause appreciable errors in the readings of the sensitive measuring devices. This difficulty of direct coupling transients cannot be overcome simply by increasing the speed of the conventional commutator, or by widening the gaps between commutator segments.

(2) There are regions where .the ground at or near the surface is such a good conductor that the skin effect becomes an important factor for frequencies of the order of 40 cycles per second. At these places it is not sufllcient to increase the electrode spacing and the sensitivity of the indicating devices to make deep measurements, but

- the frequency of commutation must also be decreased to such low value that the penetration of the current into the ground is not' nullified by the skin effect. For example, at one location in Texas, measurements made with a 37 cycles frequency showed, at a depth of about 200 feet, a

perfect insulator layer which could not be penetrated. Measurement made at the same spot with a current having a cycle frequency showed a good conductor at that depth, which result was later checked by logs from near-by wells.

It appears therefore that the synchronous commutator described above is not well suited of the commutator segments will cause the shortcircuiting periods to assum considerable time values, which will give rise to 'a dangerous condition with regard to the current-generating devices, and will cause the measuring devices to oscillate in a manner which will preventthe possibility of accurate readings.

It is, therefore, the object of this invention to provide a commutation system wherein the shorting of the circuits connected to a synchronous commutator is prevented by automatically opening these circuits at each commutation.

It is another object of this invention to prevent transient phenomena due to direct coupling from affecting the accuracy of the measurements by automatically rendering the potential measuring circuit inoperative before the current circuit is opened, and keeping it so until after the current circuit is closed, which is done by shortcircuiting the measuring circuit for this period as well as disconnecting it from the earthed electrodes.

It is a further object of the present invention to provide a synchronous commutator suitable for geophysical exploration wherein the stability of the indicating devices is increased and polarization of the potential electrodes is prevented by means of an interrupting and short-circuiting automatic device connected in the potential cir- .cuit.

Referring to Fig, 2, which shows a schematic embodiment of the conimutating system of the present invention, the circuit of the current electrodes 23 and 24 is the same as in Fig. 1 with the following exception. The gaps between the segments 2A and 2B of the commutator ring are provided with outward projections 20A and 203,

made of an insulating material such as Bakelite, amber, textilite, etc. These projections are adapted to raise or deflect the brushes 8 and 9,

thereby opening 'the circuit of the generator 2| every tim that commutation occurs. The cura sufliciently long time for the ammeter :2 to come to its final deflection, which can be accurately read, before the needle tends to fall back I to zero at the brief opening of the circuit.

The circuit of the potential electrodes 33 and 34 of Fig. 2 difiers from that of Fig. 1 in the'iollowing respects: f

The gap between the commutator segments 1A and 1B is made somewhat wider than that between commutator segments in and 2B. -Wedges "A and B, carried by the shaft l,- are inserted within these gaps and are insulated from segments 1A and 1B. These wedges usually are made of a conducting material similar to that of the shaft i, for example, brass, bronze, etc. The ends of the wedges MA and 1013 project outside the periphery of the commutator ring TAB by an amount slightly greater than that of the insulating projections 28A and MB on the current commutator. The wedges IIIA and [08 are electrically connected by'means of conductor ll revolutions per second. A low-frequency commutated interrupted current flows therefore between theelectrodes 2i and N.

The electrodes and 3t detect both the unidirectional natural ground potential, existing be anced outby adJusting the potentiometer 85. The

- alternating potential difference between the electo each other and to a slip ring' mounted on the shaft I. Slip ring II is in contact with a brush [2, which is connected by a conductor 15 to a point between the galvanometer 3i and the potential divider of a potentiometer circuit similar .to that of Fig. 1. A second potentiometer circuit, comprising a source of E. M. F. 8!, a reversin'g switch 82 and a potential divider I5 is connected in the circuit of the potential elec-- trodes '33 and 3. The object of this second potentiometer circuit is explained in the following paragraph:

The commutated or alternating current flowing in theground between the current electrodes 23 and 24 generates an alternating potential difference between the potential electrodes 33 and 34.

'trodes 33 and 34 is impressed, through the brushes i 2 and I3 and slip rings 5 and 8, on the rotating .gap in the commutator JAB is wider, and since the wedges HIA and 10B, adapted to raise or defiect the brushes II and I5, project further outside the periphery of the commutator than projcctlons 20A and "B, the circuit of the potential electrodes is respectively opened before, and is closed after the circuit of the current electrodes. At the momentof commutation, the brushes l4 and ii are short-circuited by the electrically interconnected wedges 10A and 10B. Since these wedges are electrically connected to slip ring 14,

this resultsior example, in momentarily shortcircuiting the galvanometer 36 between brush l5,

This alternating potential difierence is commutated by means of slip rings 5 and 6 and commutator lAB in synchronism with'the commutation of current between the current electrodes 23 and 24, whereby a unidirectional potential difierence is impressed on the galvanometer 36. This-unioirectional potential 'diflerence may be, balanced out by a suitable adjustment of the potential divider 35. 7

Besides an alternating potential difference due to the commutated current passed through the ground, there exists, however, between the potential electrodes 33 and 31, a unidirectional potential difference due to the natural properties of the ground at the location where electrodes 33 and 34 are grounded. This unidirectional natural potential difierence between electrodes 33 and N is also c'ommutated by means of slip rings 5 and 5, commutator ring JAB, and is therefore impressed in the form of a commutated or alternating potential on the gaivonometer 3i. '1! high speeds of commutation, such as discussed with regard to the circuits of Fig. 1 are used, the galvanometer will not follow these alternations, and the accuracy of its readings will not be atfected by this natural ground potential. With the low speeds oi commutation used. with the circuits of Fig. 2, however, the conunut-ated natural ground. potentia will cause the gaivancineter 36 to swing lihis natural unidirectional ground po- 3 is therefore balanced out, before it can to brushes i2 and t3, byaiijusting the E5 of the second potentiometerwedge 70A, conductor 1|, slip ring" 14, brush l2, and conductor 15, while at the same time the potentiometer E. M. F. is short-circuitecl between brush it, wedge 10B, conductor H, slip ring H, brush I2, and conductor 15.

It must be pointed out that the same result cannot be achieved by simply making the gaps between segments 1A and 1B sufliciently wide to prevent their being short-circuited by the brushes II and ii at the moment of commutation, be-

, rangement of Fig. 1, their short-circuiting by merits.

between the closing of the current circuit and of brushes l4 and It at each commutation will cause a direct current from the potentiometer circuit 8| to how into the ground through electrodes 33 and 3!, and would in time cause said electrodes and the ground around them to become polarized.

It has already been stated that the action of the wedges IDA and 10B short-circuits and renders the potential circuit inoperative some time before the current circuit is opened by the action or the insulating projections 20A and 20B, and that in the same manner the potential circuit is operatively closed again some time after the current circuit has also been closed. In th s manner, the relatively large undesirable transients due to direct inductive coupling between the current and the potential circuits are not registered by the indieating device 35 and do not affect the measure- The necessary duration of the time lag the potential or measuring circuit depends on the arrangement and spacing of the electrodes and the electrode spacings up to 1500 ft. or more when the current intensity in the current circuit is about purposes. Thus, in exploring a certain tract in Texas, it was found that a change in the value.

of this time lagfrom $5 to V of a second changed the resulting readings taken at different locations by a substantially constant amount with the exception of a few locations where the observed change was considerably greater. tions where a change inthe value of the time lag resulted in a disproportionately large change in the readings obtained, were found to correspond to a known position of a large fault.

The value of the time lag may be controlled either by bringing the gaps of the commutator rings IAB and 'IAB slightly out of alignment with each other (which is, however, technically difli cult), or simply by varying the speed of the commutator, or by slightly advancing or retradin the brushes I4 and I5 of the potential commutator with regard to the brushes 8 and 8 of the current commutator.

Changing the speed of commutation chang the duration of all parts of the commutation cycle. However, in view of the slow speed of rotation used, the time, lag is the only part f the cycle which is sufficiently sensitive to small changes in speed to affect the value of the readings.

These locaby H. M. Evjen in his Patent No. 2,169,685, issued August 15, 1939.

It is understood grammatic representation of the present commutator necessary for understanding its operation. In actual practice; this commutator is constructed in a manner shown in perspective in Fig. 4. and in vertical cross-section in Fig. 5, the same parts geing indicated by the same numerals as in g. Figs. 4 and 5 show that the currentcommutator consists of two cylindrical bodies 5| and 52, mounted on the shaft I. One end of each of said bodies, namely,- the ends facing each other, are

recessed so as to form segments 53 projecting coaxially with the shaft I, as shown in Figs. 4, 6 and 8. The particular commutator shown has 3 pro- I jecting segments on each at the bodies ii and 52 (although any desired number may be used) and the segments on body 5| are therefore angularly displaced with regard to those on body 52 by that is, by 60 degrees. Brushes II and III are in contact with the non-recessed portions of the bodies 5| and 52, respectively, while brushes 8 and 8 are-made sumciently wide to contact alter- The manner in which the value of the time lag may be controlled by advancing or retarding the brushes l4 and I5 of the potential commutator, is diagrammatically shown in Fig. 3, wherein A indicates the time period during which the circuit of the potential electrodes is rendered inoperative (brushes I4 and i5 being short-circuited by wedges HIA and 10B) B indicates the time period during which the circuit of the current electrodes is open (brushes 8 and 9 being raised by the insulating projections 2llAand 20B); and C indicates the time lag between the closing of the cur rent and potential circuits. This diagram clearly shows that by rotating brushes I 4 and I5 clockwise with regard to the commutator 1AB, while keeping them always 180 apart, the time lag C may be given, within a certain range, a desired value. It must'be noted that the brushes l4 and I5 should never be rotated as far as to permit the time lag C to assume a zero or negative value, whereby the potential circuit would be closed at the same time, or before the current circuit- The time lag C should preferably be given a reasonable value sufficient to allow the direct coupling transients, caused by the closing of the current circuit, to decay sufficiently to al ow significant measurements to be made.

. and 9.

nately with the projecting segments 53 on body 5| and with those on body 52. It will therefore be seen that the non-recessed portions of bodies 5| and 52 correspond to slip rings 4 and l of Fig. 2, while the projecting segments 51 on said bodies correspond to the commutator segments 2A and 2B of Fig. 2. The interrupting insulating projection .20A and 20B of Fig. 2 are made in the form of an insulating disc 55 shown in Fig. '1,

which is mounted between the bodies 5| and 52 as shown in Figs. 4 and 5. The teeth of this insulating disc are in alignment with the radial lines defining the projecting segments 53 and project outside the circumference of the commutator segments and interrupt the current at each cornmutation'by raising or deflecting the brushes I The potential commutator, also shown in Fig. is constructed in a manner generally similar to that of the current commutator with the exception that the insulating interrupting disc 55 of Fig. 7 is replaced by a short-circuiting conductor disc 56 shown in Fig. 9. The toothed disc of Fig.

9 corresponds to the short-circuiting wedges IDA and 10B and the slip ring 14 of Fig. 2. The brush 1!, as shown in Figs. 4 and 5, is mounted in this particular case at 60 degrees, or in general at an n degrees t either brush H or. brush I5, 11. being the number of teeth on the interrupting disc of Fig. '7, soas to be in contact with one of the teeth of the interrupting disc of Fig. 9 whenever brushes H and I5 are also in contact with two diametrically opposite teeth of said disc,

It is understood that the present invention is in no way limited to any of the structural details described above, such for example, as the number of commutator segments, arrangement of brushes, etc., but pertains broadly to a system of synchronous commutation particularly adapted for geophysical exploration wherein a low frequency interrupted commutated current is made to flow in the current circuit, and the potential circuit is interrupted and'short-circulted in synchronism with the commutation occurring in the that Fig. 2 gives only a diacontrol. 1

I claim as my invention:

1. In a commutation system comprising two electrical circuits, a source of direct current in the first circuit, means for periodically reversing the direction of the current flow in the first circuit, means for interrupting said flow at each reversal, an indicating device in the second cir-' cuit, means for reversing the polarity of the second circuit in synchronism, with the-current reversals of the first circuit, and means for shortcircuiting the indicating device in synchronism" with the interruptions of the current flow in the first circuit.

2. In a synchronous commutator, a rotatable shaft, a first commutator mounted 'on said shaft and insulated therefrom, said commutator comprising a first annular electrically conductive body recessed at one end to form at least one segmental projection co-axial with the shaft, the

outside circumference of said projection coinciding with that of the non-recessed portion of the annular-body, and a second recessed annular body similar to the first, the projecting segments on thetwo bodies being directed towards,

each other and anguiarly displaced with regard to each other by a number of degrees equal to 18!] divided by the number ofsegments on either bodyian insulating disc mounted on the shaft 'cuit after each interruption being subject to between said two annular bodies and having radial projections extending slightly outside the periphery of said annular bodies along the radial lines defining the area oi saidsegmental projections lna plane perpendicular to the axis of the shaft, a brush in continuous electrical contact with the non-recessed portion of the first annular body. a brush in continuous electrical contact with the non-recessed-portion of the second annular body, two brushes sngularly displaced with regard to each other by 180 degrees and having a width sumcient to permit alternate contact with the outside periphery of the projecting segments on either annular body when the shaft is 1'0- tated, asecond commutator mounted on the shaft and insulated therefrom, comprising two annular recessed bodies constructed and mounted on the shaft in a manner similar to that of the first commutator, an electrically conductive disc mounted on said shaft between said last annuiar bodies and insulated therefrom, said disc hay-- ing radial projections extending outside the periphery of said bodies by an amount slightiy greater than that of the insulating disc of the first commutator, said projections extending along radial lines defining the area of the-seg-, mental projections of said last annular bodies,

. the projections on said conductive disc being in radial alignment with the projections on them-- sulating disc of the first commutator, a brush in" continuous electrical contact with the non-recessed portion of one of said last annular bodies, a brush in continuous contact with the non-recessed portion of the other of said last annular bodies, two brushes displaced with regard to each other by 180 degrees and having a width sumcient to permit alternate contact with the outside periphery of the projecting segments on either of'said last annular bodies and with the 3. In a commutation system adapted for geo-' physical exploration, two electrical circuits each comprising the ground as a part thereof, a source of direct current in the first circuit, an indicating device in the second circuit, means for periodicaliy reversing the direction of the current flow in the first circuit, means for interrupting said flow at each reversal, means for reversing the polarity of thevsecond circuit in synchronism with the reversals of the current in the first circuit, means for opening that portion of the second circuit comprising the ground, and for simultaneously short-circulting that portion of the second circuit comprising the indicating device in synchronism with the interruptions of current occurring-in the first circuit, the beginning and end of said opening and short-circuiting periods occurring respectively before and after the beginning and end of the interruption periods in the first circuit. a

4. In the system of claim 3, means to control the relative values of the time lag occurring between the beginning of the opening and shortcircuiting period in the second circuit and the beginning of the interruption period in the first circuit, and of the timevlag occurring between the end of the interruption'period in the first circuit and the end of the opening and short-clrcuiting period of the second circuit,

5. In a commutation system adapted for geophysical exploration, a source of direct'current, a rotatable shaft, a commutator having at least two commutating segments mounted on said shaft and insulated therefrom, each of said segments being connected to a. slip ring mounted on said shaft and insulated therefrom, at least one electrical circuit connected between said slip rings,

said circuit comprising at least two grounded electrodes and the ground therehetween, means to bring each of said commutator segments into alternate contact with the terminals of the source of direct current by rotating the shaft, whereby the direction of the current flowing through the ground is periodically reversed. means rotatable with the shaft to interrupt said current at. each reversal, 2. second commutator having an equal number of commutating segments mounted on the sheft and insulated therefrom, each of said segments being connected to a slip ring mounted on the shaft and insulated therefrom, at least one electrical circuit connected between said slip rings, said circuit comprising two grounded electrodes and the ground therebetween, an indicating device, means to bring each of the segmerits of the second commutator into alternate contactwith the terminals of said indicating device, means rotatable with the shaft for simul- 6. In a commutation system adapted for geophysical exploration, a source of direct current,

a rotatable shaft, a commutator having at least two commutating segments mounted on said shaft and insulated therefrom, each of said segments being connected to a slip ring mounted on said shaft and insulated therefrom, at least one electrical circuit connected between said slip rings, said circuit comprising at least two grounded electrodes and the ground therebetween, means to bring each or said commutator segments into alternate contact with the terminals of the source of direct current by rotating the shaft, whereby the direction 01' the current flowing through the ground is periodically reversed, means rotatable with the shaft to incuit comprising a second. adjustable source 01' electromotive force and an indicating device in series between two terminals, means to bring each of the segments of the second commutator into alternate contact with said terminals, and means rotatable with the shaft and electrically connected to a point between said second source of electromotive force and said indicating device for opening the circuit oi. the grounded electrodes and for separately short-oircuiting said second source of electromotive force and said indicat-.

ing device in synchronism with the interruptions of-current occurring in the circuit of the first commutator, the beginning and end oi said opening and short-circuiting periods occurring respectively before and after the beginning and the end 0! said interruption periods.

7. In the system of claim 5, means to control the relative values of the time lag occurring between the beginning of the short-circuiting period in the second circuit and the beginning 01' the interruption period in the first circuit, and.

of the time lag occurring between the end of the interruption period in the first circuit and the end 01 the short-circuiting period of the second circuit.

and said interruption periods, and of the time lag between the end of said interruption periods and the end of said'short-circuiting periods.

10. In a method of geophysical -exploration, the steps of passing an interrupted commutated reversing current through the ground, detecting the reversing potential generated in the ground by said current, oommutating saidreversing potential in synchronism with the commutations or the current passed through the ground, impressing said commutated potential on an indicating device, reducing the readings of said-device to zero by applying thereto an opposing potential from an. adjustable source of electromotivei'orce and short-circuiting the indicating device in synchronism with the interruptions in the flow of ground.

11. In a method of geophysical exploration, the steps of causing an interrupted commutated current to flow through the ground, detecting the reversing potential generated in the ground by said current, commutating said reversing po- 8. In a method or geophysical exploration comprising the use of 'at least two current and two potential electrodes, the steps of passing an interrupted commutated current through the ground between the current electrodes, detecting between the potential electrodes the natural ground potential and the reversing potential generated by the commutated current flowing between the current electrodes, balancing out the natural ground potential by means of an adjustable source of electromotive i'orce. connected in series with one of the potential electrodes, commutating the reversing potential generated between the potential electrodes in synchronism with the commutations of the current passed through the ground, impressing said commutated tential in synchronism with the commutations oithe interrupted current flowing through the ground, impressing said commutated potential on an indicating device, and short-circuiting the indicating device in snychronism with the interruptions of the current flowing through the ground.

12. In a method of geophysical exploration by means of a system comprising a current circuit and a potential circuit, the steps of causing by means of the current circuit an interrupted commutated current to flow through the ground, de-

tecting by means of the potential circuit the unidirectional natural ground potential and the reversing. potential generated in the ground by the commutated current flowing in the current circuit, balancing out said unidirectional potential by applying an adjustable potential to the potential circuit, commutating said reversing potential-in synchronism'with the commutations of the current flowing in the current circuit, impressing said commutated potential on an indicating device 'and short-circuiting said indicating device in synchronism with the interruptions of the current flowing in the current circuit.

13. In a commutation system comprising two electrical circuits, a source of direct current in the first circuit, means Tor periodically reversing the direction of the current flow n the first circuit, anindicating device in the second circuit, means for reversing the polarity of the second circuit in synchronism with the current reversals -01 the first circuit, and means for short-circuiting the indicating device in synchronism with the reversals oi the current flow in the first circuit.

14. In a method 01' geophysical exploration, the steps of passing a commutated reversing current through the ground, detecting the reversing potential on an indicating device, and short circuiting said indicating device in synchronism with the interruptions of the current passing between the current electrodes, said short-circuiting periods beginning and ending respectively before and after said interruption periods,

. 9. In the process of claim 8, the step of varying the relative values of the time lag between the beginning of said short-circuiting periods potential generated in the ground by said current, commutating said reversing potential in synchronism with the commutatinos of the current passed through the ground, impressing said commutated potential on an indicating device, reducing the readings of said device to zero by applying thereto an opposing potential from an adjustable source of electromotive force and short-circuiting the indicating device in synchronism with the reversals in the fiow of the current passed-through the ground.

15. In a method of geophysical exploration, the steps of causing a commutated reversing curthe current passed through the reversing potential generated in the ground by said current, commutating said reversing potential in synchronism with the commutations oi the reversing current flowing through the ground. impressing said commutated potential on an indicating device, and short-circuiting said indicating device in synchronism with the rever- -sais oi. the current flowing through the ground.

16. In a method 01' geophysical expiorationby means or a system comprisingla current circuit and a potential circuit; the steps oi causing hy' means oi the current circuit a commutated reversing current in now through the ground, de-

g 21,914 7 rent to flow through the ma. detecting the tecting by means or the potential circuit the unidirectional natural ground potential and the reversing potential generated in the ground by the commutated current'flowing in the current 5 circuit, balancing out said unidirectional potential by applying anadiuatable potential to the potential circuit, commutating said reversing potential in synchronism with the commutations oi the current flowing in the current circuit, impressing said commutated potential on an indim cating device, and short-circuiting said indicating device in aynchronism-with the reversals oi the current flowing. in the current circuit. a

7 DAVID BAVILLE MUZZEY, Jn. 

